1. Field of the Invention
The present invention relates generally to a portable radio transceiver system, and more specifically to such a system which is provided with an improved adaptor for a portable radio transceiver and/or improved receive signal control arrangements.
2. Description of the Prior Art
A portable radio transceiver, such as a portable radiotelephone or the like, is designed with primary importance placed on transportability and, accordingly, is required to be small in size and light in weight.
A mobile transceiver is susceptible to so-called fading which is the variation of radio field intensity caused by changes in the transmission medium, and transmission path, with time. In order to minimize the effects of fading, a diversity scheme has been proposed. The diversity scheme is a method that is used to develop information from several signals transmitted over independently fading paths.
In order to obtain wider usage of a portable radiotelephone, viz., enable the use both outside and inside of an automotive vehicle or the like, it is a current tendency that a portable radio transceiver is designed to be attachable to a vehicle mounted adaptor when used in a passenger compartment of a vehicle.
However, a conventional vehicular adaptor is designed simply for coupling a portable radio transceiver to a car battery, a handset provided in the vehicle compartment, etc. More specifically, no prior art technique has suggested the concept of equipping a vehicular adaptor of the above mentioned type with circuitry for diversity reception.
According to a known technique, an arrangement for diversity reception is provided in the portable radio transceiver itself. However, this results in an undesirable increase in size and weight of the device.
Further, it is a common practice to provide a mobile telephone (i.e. portable radio transceiver or radiotelephone as it will be referred to hereinafter) with an indicator which shows the strength of the signal being received. An input signal to the indictor may further be used to indicate whether or not a portable radiotelephone is within a service area or not. However, a known portable radiotelephone is only provided with a single threshold level with which the receive signal strength is compared to indicate magnitude of electric field in which it is located. As a result, the known portable radiotelephone has encountered the problem that the receive signal strength varies notably between a portable mode and an in-car (vehicle mounted) mode.
This difficulty is accentuated when a portable radiotelephone is coupled to a vehicular adaptor. That is to say, upon a portable telephone being set into the adaptor, a car battery energizes some portions of the portable telephone such as a radio frequency (RF) amplifier, a mixer, etc. Since a car battery voltage is higher than a rechargeable battery installed in the portable telephone, the indicator undesirably exhibits a higher value irrespective of the fact that the electric field strength has not changed. (By way of example, a car battery is 12.5 volts while a rechargeable battery 6 volts.)